Films for use in stretch-packaging

ABSTRACT

The present invention relates to a film for use in stretch-packaging that comprises an intermediate layer which contains as its main component an α-olefin-series resin component containing a butene-1-series resin and at least two surface layers each containing an ethylene-vinyl acetate copolymer resin as its main component, the surface layers being laminated over both surfaces of the intermediate layer. The film has remarkably high values of tear strength and recovery from deformation.

TECHNICAL FIELD

The present invention relates to a film for use in the stretch-packagingof foodstuffs.

BACKGROUND ART

In recent years, so-called pre-package wherein foodstuffs such asvegetables and fruits, fresh fishes, fresh meats and daily dishes withor without a plastic tray for carrying the foodstuffs arestretch-packaged with a film is common practice. Active research iscontinued to develop, as the film for use in the pre-package, films madeof ethylene-series resins such as low-density polyethylene resins,ethylene-vinyl acetate copolymer resins, etc., in place of theconventional films made of poly(vinyl chloride), principally because ofsafety and hygienic reasons.

However, the films made of low-density polyethylene-series resins suchas low-density polyethylene resins, ethylene-butene-1 copolymer resins,etc., which have already been known as films for use instretch-packaging, are rigid and have low elongation. If the films arestretched with great force, the films will either tear or at mostexhibit uneven elongation. In some cases, the tray carrying thefoodstuff to be packaged will be deformed or collapse to producewrinkles in the film and, as a result, such the troubles that thetightening force which is necessary for accomplishing the desiredpackaging is not attainable or the packaged foodstuff becomes unsaleableare caused.

The films made of ethylene-vinyl acetate copolymer resins are free fromthe aforementioned troubles of the films made of low-densitypolyethylene-series resins if factors such as the content of vinylacetate, the melt flow rate, etc., are properly controlled. However, ifthe foodstuff to be packaged has sharp corners or if the corners of thetray carrying the foodstuff are sharp, the film made of ethylene-vinylacetate copolymer resins will tear because of the sharp corners while itis stretched in an attempt to package the foodstuff. In addition, asmall tear that may occur in the film if it contacts various objectswhich are encountered during the distribution of the stretch-packagedfoodstuff will propagate either to produce a large tear or to cause thecomplete unpackaging of the foodstuff.

The troubles of rigidity and low elongation can also be eliminated bylaminating the film of low-density polyethylene-series resin with thefilm of ethylene-vinyl acetate copolymer resin, but the other troublewhich is associated with the tear which may occur during packagingoperations and in the process of distribution will not be completelysolved, particularly when the thickness of the laminated film iscomparatively small.

With a view to eliminating the aforementioned troubles of theconventional films for use in the stretch-packaging of foodstuffs and inorder to provide a thin film suitable for use in stretch-packaging thathas appropriate degrees of lubricity and self-adhesion property andsatisfactory levels of heat sealability and transparency and which yetexhibits high degrees of stretchability and flexibility, particularlyhigh tear strength, the present inventors previously invented a film foruse in stretch-packaging which comprises a layer that contains aspecified propylene-α-olefin random copolymer resin as its maincomponent and at least two layers each containing as its main componentan ethylene-vinyl acetate copolymer resin with a vinyl acetate contentranging from 5 to 25 wt%, the latter layers being laminated over bothsurfaces of the former layer. (This invention was described in thespecification of Japanese Patent Application No. 167566/84.)

This film, however, does not fully satisfy the following requirements ofmarketability concerning recovery properties from deformation: that theproduct of interest is tightly packaged with the film and the filmprovides good fit to the product's shape without producing any wrinkles;and that even if the film is deformed because of the contact with otherobjects while the packaged product is being transported or displayed atstore or in showcases, the film is restored from the deformed statewithout producing any wrinkles.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished in order to provide a filmfor use in the stretch-packaging of foodstuffs, having the improvedrecovery properties from deformation. The principal object, therefore,of the present invention is to provide a film that is highly adaptableto automatic stretch-packaging in that it has appropriate degrees oflubricity and self-adhesion property and satisfactory levels of heatsealability while retaining high heat resistance and transparency and inthat it yet exhibits high degrees of stretchability and flexibility, aswell as remarkably high levels of tear strength and recovery fromdeformation.

The film of the present invention which is intended for use instretch-packaging comprises an intermediate layer that contains as itsmain component an α-olefin-series resin component containing abutene-1-series resin and at least two surface layers each containing asits main component an ethylene-vinyl acetate copolymer resin having avinyl acetate content of 5 to 25 wt%, the surface layers being laminatedover both surfaces of the intermediate layer.

It is essential that the resin component in the intermediate layer inthe stretch-packaging film of the present invention should contain abutene-1-series resin.

The term "butene-1-series resin" as used herein includes both homo- andcopolymers of butene-1 such as polybutene-1 and butene-1-ethylenecopolymer with an ethylene content of 1.5 to 10 wt%, which arestereoregular and have high molecular weight. These butene-1-seriesresins preferably have densities ranging from 0.89 to 0.92 g/cm³,particularly from 0.895 to 0.910 g/cm³, and melt flow rates (hereinafterabbreviated as MFR) at 190° C. within the range of 0.1 to 10 g/10 min.

An α-olefin-series resin other than the butene-1-series resin ispreferably compounded in the resin component of the intermediate layercontaining the aforementioned butene-1-series resin for the purposes ofcontrolling such factors as the film stretchability and flexibility andof allowing the film to be cut easily on a line of perforations. Theα-olefin-series resin which may be compounded for these purposes is atleast one member selected from the group consisting of apropylene-α-olefin random copolymer resin which is a random copolymer ofpropylene and at least one α-olefin selected from among ethylene and theα-olefins having 4 to 8 carbon atoms such as butene-1, hexene-1,4-methylpentene-1, octene-1, etc., and which has an α-olefin content of4 to 19 wt%, a density not higher than 0.900 g/cm³ and a MFR of 1 to 10g/10 min. at 230° C.; an ethylene-α-olefin copolymer resin which is acopolymer of ethylene and at least one α-olefin selected from among theα-olefins having 3 to 8 carbon atoms such as propylene, butene-1,hexene-1, 4-methylpentene-1, octene-1, etc., and which has an α-olefincontent of 6 to 16 wt% and a MFR of 0.1 to 5 g/10 min. at 190° C.; andan ethylene-vinyl acetate copolymer resin having a vinyl acetate contentof 5 to 25 wt%. These α-olefin-series resins may be compounded inamounts ranging from 10 to 90 wt% of the combination with thebutene-1-series resin, and it is preferable that the resin component inthe intermediate layer consists of 50 to 85 wt% of the butene-1-seriesresin and 50 to 15 wt% of the α-olefin-series resin other than thebutene-1-series resin. It is particularly preferable to use apropylene-ethylene-butene-1 copolymer resin in combination with anethylene-vinyl acetate copolymer resin.

In addition to these resin components, a rubber made of anethylene-propylene copolymer, ethylene-butene-1 copolymer, etc., andadditives to be described later may be optionally compounded in theintermediate layer for the purpose of controlling such factors asstretchability, flexibility, etc.

The reasons that the intermediate layer in the film of the presentinvention contains the butene-1-series resin as its main component areas follows. If a conventional ethylene-α-olefin copolymer resin is usedinstead of the butene-1-series resin, the resulting film having layersof an ethylene-vinyl acetate copolymer resin on both surfaces has suchlow levels of tear strength and recovery from deformation that it is notsuitable for use in stretch-packaging. If a conventionalpropylene-α-olefin random copolymer resn is used instead of thebutene-1-series resin, the resulting laminated film is relatively low instretchability and flexibility and is unable to exhibit adequaterecovery from deformation.

The intermediate layer which contains as its main component anα-olefin-series resin component containing the aforementionedbutene-1-series resin is sandwiched between layers of an ethylene-vinylacetate copolymer resin which may be selected from among the resins thatare commonly employed in stretch-packaging films. The vinyl acetatecontent of this resin ranges from 5 to 25 wt%, preferably from 10 to 20wt%, and the MFR of the resin generally ranges from 0.3 to 5 g/10 min.,preferably from 1 to 3 g/10 min., at 190° C.

If the vinyl acetate content of the ethylene-vinyl acetate copolymerresin is less than 5 wt%, the resulting laminated film has insufficientstretchability and flexibility and does not have adequate self-adhesionproperty. If the vinyl acetate content exceeds 25 wt%, the laminatedfilm has insufficient strength and does not provide an appropriatedegree of lubricity. If the MFR at 190° C. of the ethylene-vinyl acetatecopolymer resin is less than 0.3 g/10 min., the laminated film has lowelongation. If the MFR at 190° C. exceeds 5 g/10 min., the film hassmall breaking strength. In other words, sufficient film stretchabilityis not attained if the MFR at 190° C. is less than 0.3 g/10 min. or morethan 5 g/10 min.

An aliphatic alcohol-series fatty acid ester compound made by reactingan aliphatic alcohol having 1 to 12 carbon atoms, preferably 1 to 6carbon atoms, with a fatty acid having 10 to 22 carbon atoms, preferably12 to 18 carbon atoms may be added in the intermediate layer and thesurface layers in order to provide them with appropriate degrees oflubricity and self-adhesion property, as well as anti-fogging andantistatic properties. Specific examples of such esters include:monoglycerin oleate, polyglycerin oleate, glycerin triricinoleate,glycerin acetyl ricinoleate, methylacetyl ricinoleate, ethylacetylricinoleate, butylacetyl ricinoleate, propylene glycol oleate, propyleneglycol laurate, pentaerythritol oleate, polyethylene glycol oleate,polypropylene glycol oleate, polyoxyethylene glycerin, polyoxypropyleneglycerin, sorbitan oleate, sorbitan laurate, polyethylene glycolsorbitan oleate, polyethylene glycol sorbitan laurate, etc. Apolyalkylene ether polyol such as polyethylene glycol, polypropyleneglycol, etc., may be added in the intermediate layer and the surfacelayers. At least one of these esters and polyalkylene ether polyols maybe added in an amount ranging from 0.5 to 5 wt%, preferably from 1 to 3wt%.

The reasons that in accordance with the present invention, theintermediate layer which contains as its main component anα-olefin-series resin component containing the aforementionedbutene-1-series resin must be sandwiched between the surface layers eachcontaining the aforementioned ethylene-vinyl acetate copolymer resin asits main component are as follows. A film solely made of the layer of anα-olefin-series resin containing the butene-1-series resin does not havethe desired degrees of stretchability and flexibility. On the otherhand, a film solely made of the layer of an ethylene-vinyl acetatecopolymer resin has insufficient tear strength.

The stretch-packaging film of the present invention may be formed fromthe aforementioned two resin components by kneading the respective resincomponent in the separate extruder and laminating the compositionsaccording to any of the known techniques such as coextrusion, extrusionlaminating, etc., using the inflation molding or T-die molding. A filmwherein the mechanical strength (e.g., tear strength) in machinedirection is in balance with the strength in transversal direction ispreferably fabricated by coextrusion using the inflation molding, withthe blow ratio being preferably selected from the range of 3 to 7.

The film of the present invention has a thickness within the range of 8to 30 μm, with the intermediate layer having a thickness of 2 to 10 μmand each of the surface layers being 3 to 10 μm thick.

Because of the butene-1-series resin present in the intermediate layer,the stretch-packaging film of the present invention exhibitssatisfactory degrees of stretchability and flexibility, retains highheat resistance upon heat sealing, and displays particularly high levelsof tear strength and recovery from deformation. In addition, the layerof ethylene-vinyl acetate copolymer resin with a vinyl acetate contentof 5 to 25 wt% having satisfactory degrees of stretchability andflexibility which is laminated over both surfaces of the intermediatelayer serves to provide the film not only with appropriate degrees oflubricity and self-adhesion property but also with satisfactory heatsealability.

BEST MODE FOR CARRYING OUT THE INVENTION Example 1

A resin composition consisting of 70 wt% of polybutene-1 (density, 0.915g/cm³ ; MFR at 190° C., 1.8 g/10 min. (MO200 of Shell Chemical Co.)); 15wt% of propylene-ethylene-butene-1 random copolymer resin (ethylenecontent, 2.0 wt%; butene-1 content, 13.0 wt%; density, 0.896 g/cm³ ; MFRat 230° C., 5.0 g/10 min.), and 15 wt% of ethylene-vinyl acetatecopolymer resin (vinyl acetate content, 15 wt%; MFR at 190° C., 2.0 g/10min.) was kneaded in an extruder (barrel dia., 65 mm; L/D ratio, 25) at185° C. A resin composition consisting of 98.5 wt% of ethylene-vinylacetate copolymer resin (vinyl acetate content, 15 wt%; MFR at 190° C.,2.0 g/10 min.) and 1.5 wt% of monoglycerin oleate (Rikemal OL 100 ofRiken Vitamin Oil Co., Ltd.) was kneaded in an extruder (barrel dia., 50mm; L/D ratio, 25) at 160° C. The two blends were fed into a single unitof annular three-layer forming die and subjected to inflation molding(die temperature, 185° C.; blow ratio, 5.0) in such a manner that asurface layer 4 μm thick which contained the ethylene-vinyl acetatecopolymer resin as its main component was laminated on both surfaces ofan intermediate layer 5 μm thick which contained the polybutene-1containing resin component as its main component. A film having a totalthickness of 13 μm which was suitable for use in stretch-packaging wasobtained.

Meat, fish and vegetable were placed on trays of foamed polystyrene,stretched polystyrene and impact-resistant polystyrene, respectively,and were packaged with the film using an automatically stretch-packagingmachine, Model A18X of Fuji Pack System Co. The properties of the filmand the results of packaging with it are shown in the table.

Example 2

A stretch-packaging film was fabricated as in Example 1 except thatpolybutene-1 was replaced by M8640 of Shell Chemical Co. having adensity of 0.908 g/cm³ and a MFR of 1.0 g/10 min. at 190° C. Theproperties of the film and the results of stretch-packaging with it areshown in the table.

Example 3

A stretch-packaging film was fabricated as in Example 1 except that anintermediate layer was formed of a resin composition consisting of 70wt% of polybutene-1 which was of the same type as used in Example 1, 15wt% of a propylene-ethylene random copolymer resin (ethylene content;5.5 wt%, MFR, 2.2 g/10 min. at 230° C.), and 15 wt% of an ethylene-vinylacetate copolymer resin of the same type as used in Example 1. Theproperties of the film and the results of stretch-packaging with it areshown in the table.

Example 4

A stretch-packaging film was fabricated as in Example 1 except that anintermediate layer was formed of a resin composition consisting of 51wt% of polybutene-1, 20 wt% of propylene-ethylene-butene-1 randomcopolymer resin, and 29 wt% of ethylene-vinyl acetate copolymer resin,all resin components being of the same type as used in Example 2. Theproperties of the film and the results of stretch-packaging with it areshown in the table.

Example 5

A stretch-packaging film was fabricated as in Example 1 except that anintermediate layer was formed of a resin composition consisting of 60wt% of polybutene-1 of the same type as used in Example 2, 10 wt% ofbutene-1-ethylene copolymer resin (M 8010 of Shell Chemical Co.;ethylene content, 3.0 wt%; density, 0.895 g/cm³ ; MFR, 0.2 g/10 min. at190° C.), and 15 wt% each of propylene-ethylene-butene-1 randomcopolymer resin and ethylene-vinyl acetate copolymer resin which were ofthe same types as used in Example 2. The properties of the film and theresults of stretch-packaging with it are shown in the table.

Example 6

A stretch-packaging film was fabricated as in Example 1 except that anintermediate layer was formed of a resin composition consisting of 42wt% of polybutene-1 and 50 wt% of propylene-ethylene-butene-1 randomcopolymer resin, each being of the same type as used in Example 2, aswell as 5 wt% of ethylene-butene-1 copolymer resin (butene-1 content,9.0 wt%, MFR, 2.0 g/10 min. at 190° C.), and 3 wt% of monoglycerinoleate. The properties of the film and the results of stretch-packagingwith it are shown in the table.

Example 7

A stretch-packaging film was fabricated as in Example 1 except that anintermediate layer was formed of a resin composition consisting of 20wt% of polybutene-1, 70 wt% of propylene-ethylene-butene-1 randomcopolymer resin, 8 wt% of ethylene-vinyl acetate copolymer resin, and 2wt% of monoglycerin oleate, each resin component being of the same typeas used in Example 2. The properties of the film and the results ofstretch-packaging with it are shown in the table.

Example 8

A stretch-packaging film was fabricated as in Example 1 except that theintermediate layer had a thickness of 3 μm while each of the surfacelayers was 5 μm thick. The properties of the film and the results ofstretch-packaging with it are shown in the table.

Comparative Example 1

A stretch-packaging film was fabricated as in Example 1 except that theintermediate layer was solely made of an ethylene-butene-1 copolymerresin of the same type as used in Example 6. The properties of the filmand the results of stretch-packaging with it are shown in the table.

Comparative Example 2

A stretch-packaging film was fabricated as in Example 1 except that theintermediate layer was solely made of a propylene-ethylene-butene-1random copolymer resin which was of the same type as used in Example 1.The properties of the film and the results of stretch-packaging with itare shown in the table.

                                      TABLE                                       __________________________________________________________________________           Properties of Film                                                                                  Tear   Permanent                                        Tensile Tensile                                                                              Tensile                                                                              Strength in                                                                          Elongation                                       Elastic Breaking                                                                             Elongation                                                                           Elmendorf                                                                            after 100%                                       Modulus Strength                                                                             at Break                                                                             Test   Stretching                                                                          Apptitude for Packaging                    (kg/cm.sup.2)                                                                         (kg/cm.sup.2)                                                                        (%)    (kg/cm)                                                                              (%)   Tensile  Tear  Wrinkle              Run No.                                                                              MD*                                                                              TD** MD TD  MD TD  MD TD  MD TD Stretchability                                                                         Formation                                                                           Formation            __________________________________________________________________________    Example 1                                                                             930                                                                             1250 430                                                                              380 220                                                                              490 120                                                                              80  14 11 good     no tear                                                                             no wrinkle           Example 2                                                                            1020                                                                             1490 410                                                                              390 230                                                                              500 190                                                                              85  14 12 "        "     "                    Example 3                                                                             900                                                                             1220 410                                                                              340 200                                                                              470  95                                                                              70  17 15 "        "     "                    Example 4                                                                             860                                                                             1120 350                                                                              340 210                                                                              460  90                                                                              69  15 13 "        "     "                    Example 5                                                                             930                                                                             1200 420                                                                              380 200                                                                              480 190                                                                              77  14 12 "        "     "                    Example 6                                                                            1650                                                                             1520 360                                                                              310 280                                                                              530  85                                                                              95  19 17 "        "     very few                                                                      wrinkles             Example 7                                                                            1850                                                                             1700 340                                                                              310 320                                                                              500  40                                                                              140 24 20 "        "     local fine                                                                    wrinkles             Example 8                                                                             850                                                                              930 350                                                                              300 240                                                                              470  90                                                                              66  10  9 "        "     no wrinkle           Comparative                                                                          1200                                                                             1200 270                                                                              320 400                                                                              610  55                                                                              58  30 25 "        occasional                                                                          positive             Example 1                                          tear                       Comparative                                                                          2100                                                                             2400 360                                                                              290 350                                                                              540  30                                                                              180 36 31 rigid, hard to                                                                         no tear                                                                             some                 Example 2                                 stretch, un-   wrinkles                                                       evenly stretched                    __________________________________________________________________________     MD*: Machine direction                                                        TD**: Transversal direction                                              

The values of "permanent elongation" indicated in the table weremeasured by the following procedures: a test piece having a width of 10mm and a gage length of 50 mm was stretched by 100% at a rate of 200mm/min. and held in the stretched state for 1 minute; the load wasremoved and the permanent elongation remaining 5 minutes later wasmeasured.

INDUSTRIAL APPLICABILITY

The film for use in stretch-packaging of the present invention haveproperties that meet all of the requirements for use instretch-packaging. In addition, the film exhibits remarkably high valueof tear strength and recovery from deformation and, hence, will findutility in prepackage applications.

What is claimed is:
 1. A film for use in stretch-packaging, whichcomprises: an intermediate layer that has as its main component anα-olefin-series resin component containing a butene-1-series resin andat least two surface layers each containing as its main component anethylene-vinyl acetate copolymer resin with a vinyl acetate content of 5to 25 wt%, the surface layers being laminated over both surfaces of theintermediate layer.
 2. The stretch-packaging film according to claim 1,wherein the α-olefin-series resin component in the intermediate layerconsists of 10 to 90 wt% of the butene-1-series resin and 90 to 10 wt%of other α-olefin-series copolymer resin.
 3. The stretch-packaging filmaccording to claim 1, wherein the α-olefin-series resin component in theintermediate layer consists of 50 to 85 wt% of the butene-1-series resinand 50 to 15 wt% of other α-olefin-series copolymer resin.
 4. Thestretch-packaging film according to claim 2 or 3, wherein said otherα-olefin-series copolymer resin is a propylene-α-olefin random copolymerresin which is a copolymer of propylene and at least one α-olefinselected from among ethylene and the α-olefins having 4 to 8 carbonatoms, the content of said α-olefin being in the range of 4 to 19 wt%.5. The stretch-packaging film according to claim 2 or 3, wherein saidother α-olefin-series copolymer resin is an ethylene-α-olefin copolymerresin which is a copolymer of ethylene and at least one α-olefinselected from among the α-olefins having 3 to 8 carbon atoms, thecontent of said α-olefin being in the range of 6 to 16 wt%.
 6. Thestretch-packaging film according to claim 2 or 3, wherein said otherα-olefin-series copolymer resin is a mixture of a propylene-α-olefinrandom copolymer resin and an ethylene-vinyl acetate copolymer resinwith a vinyl acetate content of 5 to 25 wt%, said random copolymer resinbeing a copolymer of propylene and at least one α-olefin selected fromamong ethylene and the α-olefins having 4 to 8 carbon atoms, the contentof said α-olefin being 4 to 19 wt%.
 7. The stretch-packaging filmaccording to claim 2 or 3, wherein said other α-olefin-series copolymerresin is a mixture of an ethylene-α-olefin copolymer resin and anethylene-vinyl acetate copolymer resin with a vinyl acetate content of 5to 25 wt%, said ethylene-α-olefin copolymer resin being a copolymer ofethylene and at least one α-olefin selected from among the α-olefinshaving 3 to 8 carbon atoms, the content of said α-olefin being 6 to 16wt%.
 8. The stretch-packaging film according to claim 1, wherein saidintermediate layer and each of said surface layers contains 0.5 to 5 wt%of an aliphatic alcohol-series fatty acid ester compound which is thereaction product of an aliphatic alcohol having 1 to 12 carbon atoms anda fatty acid having 10 to 22 carbon atoms.
 9. The stretch-packaging filmaccording to claim 1, wherein the film product has a thickness withinthe range of 8-30 μm, the intermediate layer of said film ranges inthickness from 2-10 μm, and each of the surface layers of the film rangein thickness from 3-10 μm.
 10. The stretch-packaging film according toclaim 1, wherein said butene-1-series resin has a density ranging from0.89 to 0.92 g/cm³ and a melt flow rate at 190° C. within the range of0.1 to 10 g/10 min.
 11. The stretch-packaging film according to claim10, wherein said butene-1-series resin has a density ranging from 0.895to 0.910 g/cm³.
 12. The stretch-packaging film according to claim 1,wherein said ethylene-vinyl acetate copolymer resin has a vinyl acetatecontent ranging from 10-20 wt.%
 13. The stretch-packaging film accordingto claim 1, wherein said ethylene-vinyl acetate copolymer resin has anMFR ranging from 0.3-5 g/10 min at 190° C.
 14. The stretch-packagingfilm according to claim 1, wherein said MFR value ranges from 1-3 g/10min.